Friday, June 29, 2007

Concluding the series on Java garbage collection

Over the past few blog posts we have covered some basics of garbage collection in Java. Garbage collection is a key strength of the JVM, since we do not have to worry about releasing object memory. It reduces memory leaks and other problems associated with improper memory management code that often creeps in when we program against hard deadlines :-)

However, for large programs we very often have to tweak the default garbage collection mechanism to improve performance. Here's a nice article that explains garbage collection in much more detail with good examples. This page contains several links to memory management in the Java Hotspot VM. Here's another page that explains how to fine tune the Java garbage collector in Java 1.5.

It is also a good idea to keep up with the latest in technology, so here's a page that describes enhancements to the Java VM in version 1.6, that influence the garbage collector.

I hope you enjoyed this series. Over the next few months I hope to cover various aspects of core Java through such mini series posts. I hope you find them informative. As always your comments and suggestions are very welcome.

Note: This text was originally posted on my earlier blog at http://www.adaptivelearningonline.netHere are the comments from the original post

-----COMMENT:AUTHOR: Sanket DaruURL: http://sankdadevildaru.blogspot.comDATE: 06/29/2007 05:13:19 AMDear Sir,Indeed the mini-series on JVM Garbage Collection was very informative. At the conclusion, I have one question though.

Well this might be out of scope of the mini-series, but I want some guide. I was working with XML parsing in Java and started off using DOM parsing. Now DOM has its limitations, mainly due to its HEAVY footprint and it ends up giving me "out of memory" errors... I tried several other means but nothing seemed to work out...

Eventually I left the matter because I was just trying it out for experimentation. I had decided to use SAX due to its light footprint and event-based modeling.

Now my question is, can we tweak memory management of JVM and Garbage Collector programatically? Such a solution will be really helpful.

As of now I do not believe there is a way to "programatically" tweak garbage collection. You can tweak the JVM when you start it using:-Xms -Xmx for heap size.Using various other options as described in (http://java.sun.com/docs/hotspot/gc5.0/gc_tuning_5.html), you can modify the generation sizes and ask the JVM to use a particular JVM algorithm.

However, all these things have to be done when the JVM is started.

Increasing the heap size will allow you to hold a larger DOM tree in memory, but tweaking the GC will only help you with performance, not with being able to hold more objects in memory.

One potential solution might be to use serialization. If you can hold only enough information in the nodes such that your entire tree can live in memory, and when a node is visited it's details are retrieved from a serialized object. Once you navigate away from that node release the memory (modifying the serialized object of any changes are made).

I am not sure of this is a standard way of dealing with this problem and if an API already exists to achieve it. But it seems like a plausible solution.